Are you thinking of continuing the story which were set in the anime, or an OVA with characters from the series? Either way, my list:
1. Katekyo hitman reborn
2. D.-Gray man
3. Deadman wonderland/Mirai nikki. The more psychopaths, the better it gets.

there is to many series i want to continue but top three i guess would be...

1) Claymore - cont series (gets so good needs another season)
2) Bodacious Space Pirates - cont series (movie is a + but want a second season)
3) Sekirei - cont series (needs a third season)
3) Negima - cont series (want to see the harem cont.)
3) Sasameki Koto - cont series (want to see them get together like in the manga so need a 2 and 3 season)
3) Strike Witches - cont series (movie was amazing needs a third Season)
3) The Sacred Blacksmith - cont series (want to see a second season)
3) Infinite Stratos - cont series (want to see the harem cont needs a second season and more Mech Action)
3) Highschool of the Dead - cont series (needs a second season(thank god the manga is starting back up))
3) Nura Rise of the Yokai Clan - cont series (want to see the war needs a third season)
3) Burst Angel - cont series (want to see her solo needs a second season)
3) Chrome Shelled Regios - cont series (needs a second season first was great)
3) Dance of the Vampire Bund - cont series (pretty awesome start to a Bad ass vampire series think needs second season)
3) Air Gear - cont series (want to see the tricks cont needs a second season)
3) Black Lagoon (needs a second season badly action was way good in this show)
3) Bleach (they better anime the final arc)
3) Spice and Wolf (needs third season want to see her home and family)

and that's my top Three and i'm sure there are more but those have to be my top Three i can think of off the top of my head